1. Field of the Invention
The present invention relates to an electrically assisted power steering system for a motor vehicle such as an automobile.
2. Description of the Prior Art
Electrically assisted power steering systems for use on motor vehicles have a rack shaft which can be axially moved by a pinion meshing therewith when rotary force is applied from a steering wheel through a steering shaft to the pinion. When the rack shaft is axially moved, it causes tie rods coupled to respective ends of the rack shaft to turn road wheels operatively connected to the tie rods. The steering torque applied to the steering shaft is detected, and an electric motor is energized based on the detected value of steering torque. The drive force produced by the electric motor is transmitted to the rack shaft through a ball-and-nut mechanism for thereby giving power assist to the rack shaft in turning the road wheels.
One known electrically assisted power steering system is disclosed in Japanese laid-open patent publication 62-26153 corresponding to U.S. Pat. No. 4,666,014. In such disclosed electrically assisted power steering system, an electric motor is disposed around a rack shaft in a housing in which the rack shaft is slidably supported. Rotary force from the electric motor is transmitted through a ball-and-nut mechanism on the rack shaft.
Because the road wheels apply reactive forces to the rack shaft and the rack shaft suffers dimensional errors introduced when it is manufactured or assembled, the rack shaft tends to be flexed or displaced axially or radially during operation of the electrically assisted power steering system. The ball-and-nut mechanism, whose operation efficiency sharply decreases when subjected to small strains or displacements, cannot directly bear strains or displacements of the rack shaft. In such prior electrically assisted power steering system, the ball-and-nut mechanism is supported in the housing by two thrust bearings which allow the nut to move slightly radially depending on the flexing or strain of the rack shaft.
However, the two thrust bearings make the ball-and-nut mechanism and associated components complex in structure. It is tedious and time-consuming to assemble such ball-and-nut mechanism and associated components in combination with the two thrust bearings. The two thrust bearings also pose limitations on the layout of other members of the electrically assisted power steering system.